This invention generally relates to a method and apparatus for continuous inkjet printing, and more particularly to a continuous inkjet printing method wherein a first stream of ink droplets traveling along a first flow path is used as a mask by colliding with a second stream of ink droplets traveling along a second, intersecting flow path in route to a receiver on which an image is to be printed, selected droplets of the second droplet stream being timed to pass between and avoid the masking droplets so as to travel on and impinge the receiver for forming the image thereon.
An inkjet printer produces images on a receiver by ejecting ink droplets onto the receiver in an image-wise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of inkjet printers in the marketplace.
Inkjet printing mechanisms can be categorized as either Drop-on-Demand or continuous inkjet. Continuous inkjet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
The term xe2x80x9cDrop-on-Demandxe2x80x9d characterizes inkjet printers, wherein at every orifice a pressurization actuator is used to produce the inkjet droplet. In this regard, either one of two types of actuators may be used. These two types of actuators are heat actuators and piezoelectric actuators. With respect to heat actuators, a heater placed at a convenient location heats the ink and a quantity of the ink will phase change into a gaseous steam bubble and raise the internal ink pressure sufficiently for an ink droplet to be expelled to the recording medium. A feature of the heat-type actuators is the ability to incorporate them easily into modern known print head constructions, particularly those using silicon substrates with CMOS electrical circuitry. One disadvantage, however, is that the overall electrical power consumption is large, especially in xe2x80x9cpage-widthxe2x80x9d arrays. With respect to piezoelectric actuators, a piezoelectric material is used, which piezoelectric material possesses piezoelectric properties such that a mechanical stress is produced when an electric field is applied.
The most common of the xe2x80x9ccontinuousxe2x80x9d inkjet printers utilize electrostatic charging tunnels that are placed close to the point where ink droplets are being ejected in the form of a stream. Selected ones of the droplets are electrically charged by the charging tunnels. The charged droplets are deflected downstream by the presence of deflector plates that have a predetermined electric potential difference between them. A gutter may be used to intercept the charged droplets, while the uncharged droplets are free to strike the recording medium. A disadvantage of the known continuous inkjet printers, however, is that the charging apparatus is complex and costly to incorporate into the print head. In addition, the interaction between charged drops can adversely affect image quality.
A novel continuous inkjet printer is described and claimed in U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, and assigned to the Eastman Kodak Company. Such printers use asymmetric heating in lieu of electrostatic charging tunnels to deflect ink droplets toward desired locations on the recording medium. In this device, a droplet generator formed from a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter is provided for each of the ink nozzle bores. Periodic actuation of the heater element via a train of uniform electrical power pulses creates an asymmetric application of heat to the stream of droplets to control the direction of the stream between a print direction and a non-print direction.
While such continuous inkjet printers have demonstrated many proven advantages over conventional inkjet printers using electrostatic charging tunnels, there are still some areas in which such printers can be improved, particularly in the area of the ability to operate reliably on a wide range of different ink fluids, and in lower-temperature operation of heaters.
For example, the use of two fluid jets in droplet formation, has been disclosed by Sangiovanni et al. in U.S. Pat. No. 4,341,310 issued on Jul. 27, 1982, for a method called xe2x80x9cmaskingxe2x80x9d. In this xe2x80x9cmaskingxe2x80x9d method, separate streams of xe2x80x9cpolarxe2x80x9d and xe2x80x9cnon-polarxe2x80x9d monodispersed liquid droplets are coordinated to intersect at an intersection point to xe2x80x9cmaskxe2x80x9d or prevent passage of the xe2x80x9cnonpolarxe2x80x9d liquid droplets. This technique, however, does not involve colliding jet streams of ink in an image-wise manner for printing purposes. But rather, it requires a complex charging apparatus for altering the path of the xe2x80x9cpolarxe2x80x9d droplets. This is costly and requires a relatively high voltage, not easily compatible with known low voltage CMOS print head systems, typically operating at from two to six volts.
Therefore, there is a need to provide an inkjet printing method that provides the respective advantages of continuous inkjet printing, and Drop-on-Demand inkjet printing, with low voltage operation and low power consumption. To accomplish this by the use of a inkjet-masking concept, which avoids the complexity and cost disadvantages of the known xe2x80x9cmaskingxe2x80x9d methods would be a surprising but welcomed advancement in the art.
An object of the present invention is to provide a continuous inkjet printing method and apparatus which utilizes desirable aspects of xe2x80x9con-demandxe2x80x9d printing and xe2x80x9cmaskingxe2x80x9d concepts without including the undesirable aspects of their respective printing apparatus.
With this object in view, the present invention resides in an inkjet printing method comprising the steps of (1) generating a first stream of ink droplets traveling along a first flow path, and (2) generating a second stream of ink droplets traveling along a second flow path which intersects the first flow path at a predetermined location. The second stream of ink droplets includes ink droplets traveling in timed relation to the droplets of the first stream so as to collide with the droplets of the first stream at the predetermined location and be diverted to an ink receptacle. The second stream of ink droplets also includes selected droplets traveling in timed relation to the droplets of the first stream so as to pass between the droplets of the first stream at the predetermined location and continue along the second flow path so as to impinge a receiver at a down stream location along the second flow path for forming an image on the receiver.
According to an exemplary embodiment of the present invention, an inkjet printer is provided comprising an element for emitting a first ink stream along a first flow path; an element located along the first flow path upstream of the predetermined location for controllably breaking the first ink stream into successive ink droplets traveling along the first flow path; an element for emitting a second ink stream along a second flow path which intersects the first flow path at a predetermined location; an element located along the second flow path upstream of the predetermined location for controllably breaking the second ink stream into successive ink droplets traveling along the second flow path; and an element for controlling the time relationship of droplet formation between the ink streams such that selected ink droplets of the first stream will pass between or collide with the ink droplets of the second stream at the predetermined intersection location in an image-wise manner. In the absence of a collision between droplets, the droplets moving along the first path impinge on an image receiver located beyond the predetermined jet-crossing location.
Another feature of the present invention is the provision of an element for controllably generating a stream of ink droplets by intermittently effecting surface tension and viscosity changes in a continuous stream of ink.
Another feature of the present invention is the provision of two streams of ink droplets traveling along intersecting flow paths, wherein one of the streams of ink droplets includes selected droplets timed to pass between the droplets of another of the streams so as to travel on and impinge a receiver for forming an image thereon.
Another feature of the present invention is the provision of an element for controllably breaking a stream of ink into a succession of ink droplets traveling in timed relations to one another along a flow path.
Another feature of the present invention is the provision of streams of ink droplets generated by transiently heating continuous streams of ink to break the streams into the droplets, wherein larger ink droplets are generated by longer time intervals between the heat pulses and smaller ink droplets are generated by shorter intervals between the heat pulses.
An advantage of the present invention is the capability to selectively mask a stream of ink droplets without requiring droplet electrical polarization.
Another advantage of the present invention is the capability to generate different size ink droplets from a single continuous ink stream.
Still another advantage of the present invention is the ability to provide a drop-masking continuous ink jet printing method that is compatible with a low voltage print head system.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings which show and describe illustrative embodiments of the invention.